Ca2+ signalling behaviours of intramuscular interstitial cells of Cajal in the murine colon

Abstract

Key points: Colonic intramuscular interstitial cells of Cajal (ICC-IM) exhibit spontaneous Ca²⁺ transients manifesting as stochastic events from multiple firing sites with propagating Ca²⁺ waves occasionally observed. Firing of Ca²⁺ transients in ICC-IM is not coordinated with adjacent ICC-IM in a field of view or even with events from other firing sites within a single cell. Ca²⁺ transients, through activation of Ano1 channels and generation of inward current, cause net depolarization of colonic muscles. Ca²⁺ transients in ICC-IM rely on Ca²⁺ release from the endoplasmic reticulum via IP₃ receptors, spatial amplification from RyRs and ongoing refilling of ER via the sarcoplasmic/endoplasmic-reticulum-Ca²⁺ -ATPase. ICC-IM are sustained by voltage-independent Ca²⁺ influx via store-operated Ca²⁺ entry. Some of the properties of Ca²⁺ in ICC-IM in the colon are similar to the behaviour of ICC located in the deep muscular plexus region of the small intestine, suggesting there are functional similarities between these classes of ICC.

Abstract: A component of the SIP syncytium that regulates smooth muscle excitability in the colon is the intramuscular class of interstitial cells of Cajal (ICC-IM). All classes of ICC (including ICC-IM) express Ca²⁺ -activated Cl^- channels, encoded by Ano1, and rely upon this conductance for physiological functions. Thus, Ca²⁺ handling in ICC is fundamental to colonic motility. We examined Ca²⁺ handling mechanisms in ICC-IM of murine proximal colon expressing GCaMP6f in ICC. Several Ca²⁺ firing sites were detected in each cell. While individual sites displayed rhythmic Ca²⁺ events, the overall pattern of Ca²⁺ transients was stochastic. No correlation was found between discrete Ca²⁺ firing sites in the same cell or in adjacent cells. Ca²⁺ transients in some cells initiated Ca²⁺ waves that spread along the cell at ∼100 µm s^-1 . Ca²⁺ transients were caused by release from intracellular stores, but depended strongly on store-operated Ca²⁺ entry mechanisms. ICC Ca²⁺ transient firing regulated the resting membrane potential of colonic tissues as a specific Ano1 antagonist hyperpolarized colonic muscles by ∼10 mV. Ca²⁺ transient firing was independent of membrane potential and not affected by blockade of L- or T-type Ca²⁺ channels. Mechanisms regulating Ca²⁺ transients in the proximal colon displayed both similarities to and differences from the intramuscular type of ICC in the small intestine. Similarities and differences in Ca²⁺ release patterns might determine how ICC respond to neurotransmission in these two regions of the gastrointestinal tract.

Keywords: Ca2+ imaging; Ca2+ stores; SIP syncytium; c-Kit; colon; gastrointestinal motility; optogenetics.